1. Field of the Invention
The invention relates to a method for correcting the DC offset of a quadrature modulated RF signal.
2. Description of Related Art
Quadrature modulators are devices that are used in many digital wireless communication systems. They are used to form one signal containing a digital bit stream and a radio frequency carrier using in-phase (I) and quadrature (Q) input signals. The output is a modulated signal ready for transmission after amplification. In order to perform at their best, such modulators have to have the following three parameters (among others) under control:
a. amplitude imbalancexe2x80x94the I and Q channels must have equal amplitude gain;
b. quadrature errorxe2x80x94deviation from the ideal 90xc2x0 phase shift between the I and Q channels;
c. DC offset that is usually generated by the modulator itself and is a cause for degradation in carrier suppression. xe2x80x9cCarrier suppressionxe2x80x9d is the leakage of the un-modulated RF signal through the modulator.
The cost of a quadrature modulator is largely determined by the quality of the foregoing parameters a-c. In order to reduce the cost of a transmitter, it is common to use lower cost modulators and actively correct the above parameters a-c in order to meet specifications.
Alternatively, it may be necessary to improve the performance of existing modulator hardware to allow accommodation of new standards, such as IS 136+. Accordingly, a means was needed to determine the extent to which a given quadrature modulator exhibited the foregoing parameters a-c and, if there are errors, to correct them quickly, preferably without time consuming trial and error search procedures.
The problems described above are generally related to the test and/or calibration of both base stations and terminals such as digital and phone transmitters.
In many practical cases, the prior art technique consisted primarily of trial and error. Many TDMA and CDMA transmitters have AC and DC I and Q inputs. A technician would typically apply a variety of offset signals, either manually or using a computer program, until all of the three parameters a-c were within acceptable limits. After that point, the transmitter calibration was complete. Unfortunately, the trial and error technique sometimes requires a large number of tests to bring an RF transmitter into alignment.
One example of a prior art correction technique is described in a book entitled xe2x80x9cDigital Techniques for Wideband Receiversxe2x80x9d by J. Tsui and is set forth in Chapter 8, Section 14, thereof and entitled xe2x80x9cDigital Correction of IQ Channel Imbalancexe2x80x9d. This prior art technique uses an expression for time-domain modulator output as the starting point. By using a tone signal and comparing four measured time-domain samples to the modulator mathematical expression, it is possible to solve for the DC offset, IQ imbalance and quadrature error. This prior art method is best suited for receivers, which time-domain samples are likely to be the normal output data of the unit under test. It is not believed that this technique is especially well suited for transmitters.
In contrast, the present invention is advantageous for transmitters, where the output signal is an RF carrier. In this case, according to the prior art, specialized instrumentation, such as a vector signal analyzer or a down converter and digitizer, would be needed to acquire the time samples. According to the present invention, however, various different embodiments thereof use three, four or five measurements, but these measurements correspond to different I and Q combinations of the probe offset D. One particular advantage of the present invention, therefore, is that the measurements may be performed using the most common transmitter test instrumentsxe2x80x94the RF spectrum analyzer or RF power meter. Furthermore, two embodiments of the present invention include the scale factor(s) to convert between the measured data and the control variables.
It was in the context of the foregoing problem that the present invention arose.
Briefly described, the invention comprises a method for correcting DC offset in a quadrature modulated RF signal. According to the first embodiment, applicable under certain ideal circumstances, three measurements using known applied DC offsets are analyzed to find the DC offset corrections. According to a second embodiment, applicable under certain practical circumstances, a fourth measurement is made in order to determine an offset correction scale factor. The foregoing method is significantly more accurate and efficient than the prior art trial and error method.
The invention may be more fully understood with reference to the following drawings.